Trim Multiple Selected Elements: Automatic Trim

2.
Select the Design > Connect > Trim Elements to Roof/Shell command (or use context menu).
3.
From the Trim Elements dialog box, choose the default Use Roofs/Shells from current selection and click Trim.
Roofs and Shells trim each other so that any element parts that fall inside of the other’s trimming body are eliminated. That is, the outside parts remain; what falls inside is cut out (The intersection priority among trimmed Roofs and Shells is determined by their respective Building Materials.)